Optical amplifiers, in particular Erbium doped fiber based amplifiers (EDFAs) are currently the most commonly used optical amplification devices used to amplify optical signals which have been weakened by the attenuation of transmission fibers, and by network elements such as add/drop multiplexers, optical cross-connects, switches or routers. EDFAs are largely used for both short and long haul optical communication networks, as well as in CATV broadcasting. The newest wavelength division multiplexing (WDM) systems, in which all channels are carried simultaneously by the same fiber, require even more use EDFAs since the presence of multiple wavelengths (channels) offers great opportunity and flexibility for network planning and data/voice traffic routing.
The conventional EDFA bandwidth has been extended recently from the standard 1525-1565 nm band to a new 1565-1605 nm band. A broadband amplifier which covers the expanded band of 1525-1605 nm is described in U.S. patent application Ser. No. 09/026,657 filed Feb. 20, 1998 and entitled HIGH EFFICIENCY BANDWIDTH DOUBLED AND GAIN FLATTENED SILICA FIBER AMPLIFIER, and which is incorporated herein by reference. Therefore more bandwidth than previously has been made available to increase the number of channels which are carried, allowing optical networks to transport information at the speed of one or more terabits per second.
In an optical amplifier used in a single channel SONET/SDH system, there is little signal amplitude variation other than binary logical levels in which a signal is present or a signal is not present. The wavelength of the single channel is predetermined before the network has been installed. Amplifier response to a single wavelength is simple, as there is no competition among different channels.
On the other hand, in a Dense Wavelength Division Multiplex (DWDM) based network, in which there can be hundreds of channels, the amplitude variation of the input signal to the optical amplifier can be very large, with many thousands of combinations involving signal level and wavelength. As a result, the number of channels, or the total available optical signal input to the optical amplifier for amplification is substantially never constant. When data/voice traffic is low, only a few channels (transmitter/receiver pairs) operate. Traffic increase brings more channels into operation, and therefore the optical signal level needed for amplification is changed.
During transport, some channels may be dropped at a hub (e.g. at a town in the middle of the transport route, for example). At another hub, channels transporting traffic from a neighboring city may join the mainstream signals. This demonstrates two cases for signal level or quantity of channel change in the optical transport fiber.
When the number of channels is higher (thanks to a broader optical amplifier bandwidth), the magnitude of signal level change is also greater. For example, for a 100 channel DWDM system, signals entering an optical amplifier can change from a single channel having a weak signal (e.g. a tenth of a microwatt) to a hundred channels all at strong levels (tens of miliwatts), representing a total level change of ten thousand times.
From the point of view of the optical amplifier, the magnitude of amplification required for each channel depends not only on its input level, but also on the total number of channels (or the total composite input level). If the signal level is low (microwatts for example), large amplified spontaneous emission (ASE noise) will be generated, more at some wavelengths than at others, mixed together with the signals so that the amplifier system monitoring and gain control is affected. If the signal level is very large, the amplifier will operate in deep saturation and due to the physics of energy transfer between different atomic levels of Erbium ions, some channels will be more amplified while others are so depleted that they could experience loss instead of gain. Thus in DWDM systems, most of the amplifiers are required to operate in a constant gain mode, wherein no matter what is the input level, the amplifier pump power is adjusted to achieve always the same gain, which corresponds to a constant output per channel.
An optical amplifier used in a fiber transport system usually has a typical block diagram shown in FIG. 1. A tap optocoupler 1 takes a small portion of an input optical signal and sends it to an input photodetector 3. The main optical signal is passed through an optical amplifier 5 (shown as a gain medium) which has controllable gain. A small portion of the output optical signal from the amplifier 5 is optocoupled via a tap 7 and is sent to an output photodetector 9. The photodetectors translate the detected portions of the optocoupled signals into electronic signals.
Each of the photodetectors 3 and 9 provides its respective electronic signal to an electronic control circuit 11 (which may or may not include a microprocessor). The control circuit calculates the gain by subtracting the amplitude of the detected portion of the output optical signal from the detected portion of the input optical signal, and reports this to a central network management system via a user interface. The gain of the optical amplifier 5 can also be controlled to a desired level, by receiving control signals derived from electronic signals provided from the user interface.
The optical amplifier 5 is usually comprised of a pumping source, coupling and directional devices, active amplification material and may also contain filters, gratings, etc. to provide gain flattening over the required band.
In order for the amplifier to accept and accurately execute control commands from the central network management system, accurate detection of the input and output signal via the two photodetectors is the most critical aspect. However, when the input signal amplitude varies by an order of ten or a hundred thousand times, the input detector cannot follow the signal, and does not have sufficient resolution to detect weaker signals. The output photodetectors detect only the total optical level, regardless of whether it consists of a signal or noise. If the electronic control circuit takes everything the output detector sends, the command from the central network management system can cause the amplifier gain setting to be executed with a large error when noise content becomes large.
FIG. 2 is a graph of the difference between the desired amplifier gain against input power level. It may be seen that this difference (i.e. the error) increases from an ideal (zero error, shown as the calibration point) with input power typically due to noise, and also increases with input power level from the calibration point, typically due to resolution or narrow dynamic range and/or saturation of the input and output signal level detectors. The overall error (the difference between the desired value set by the network management system and the actual value obtained from the amplifier), results from the sum of the two causes.
FIG. 3 illustrates a photo-detection system which is commonly used as element 3 in the prior art system of FIG. 1. An optical coupler 13 provides a tap (element 1) function, and feeds an optical signal to the input of the optical section 15 of the system which includes the gain medium 5. The output signal from the optical section is applied to the input of an optical coupler 17 which provides the function of tap 7 of FIG. 1.
The optical coupler 13 provides at its output an optical signal which is detected in an input photo-detector 19, which can be comprised of a photodiode. The photodiode (its anode, as shown, or its cathode, depending on biasing polarity), is connected via a resistor 21 to a reference voltage source REF.
Similarly, the optical coupler 17 provides at its output an optical signal which is detected in an output photo-detector 23, which can be comprised of a photodiode. That photodiode (its anode, as shown, or its cathode, depending on biasing polarity), is connected via a resistor 25 to the reference voltage source REF.
An analog electrical output signal, representing the amplitude of the input optical signal, is taken from the junction of photo-detector 19 and is applied to an input of an analog to digital converter 27 in a microchip amplifier based system. Similarly, an analog electrical output signal, representing the amplitude of the optical signal which has been amplified in optical section 15, is taken from the junction of photo-detector 23 and is applied to another input of the analog to digital converter 27. The input signals to the analog to digital converter are converted to digital form and are applied to control electronics 29.
The control electronics receives an input signal N from the network administration system, indicating what is the desired gain of that stage, for the system. It divides the output signal amplitude with the input signal amplitude as indicated to the control electronics 29 by the respective signals detected from the photo-detectors via the analog to digital converter, to achieve a current amplifier gain value, and compares this with the desired gain. The control electronics then adjusts the pump power in a well known manner (not shown) to increase or decrease the gain of the optical section.
However, the optical section gain that is detected is not correct, due to ASE noise, as noted earlier. For that reason the measured gain is likely less than the real signal gain, and the amount of indicated correction will be less than what is really required.
In addition, the ASE noise changes with input optical signal amplitude, increasing with lower amplitude. Further, the noise level is different for differing wavelengths for a wideband amplifier, and for such an amplifier driven at different input signal amplitude levels at different wavelengths.
Thus the prior art detection circuit is not suitable particularly for wideband, non-saturated applications.